1. Field of the Invention
The invention relates in general to a liquid crystal on silicon (LCOS), and more particularly to a vertical alignment (VA) mode liquid crystal on silicon capable of forming a single domain.
2. Description of the Related Art
As the market of portable products, e.g., personal digital assistant (PDA), cellular phone, and projector, and large-sized projection television progress, more and more customers require that the resolutions of these portable products or the projection television are to be identical to that of personal computer systems. Liquid crystal on silicon (LCOS) is just enough to fulfil these requirements. Unlike liquid crystal display (LCD) whose the front and rear plates are made of glass, LCOS employs a silicon plate and glass plate between which liquid crystal is filled. The structure of LCOS can provide displays not only meeting the requirement for compactness of portable products, but also having high resolution. The resolution of a display is represented by pixels formed on the plates. The more the pixels a display has, the finer and the resolution the display can show. In addition, LCOS is capable of having its driving circuit manufactured by using semiconductor manufacturing process, e.g., complementary metal oxide semiconductor manufacturing process so that the silicon plate that uses silicon wafer can be manufactured in a standard semiconductor manufacturing production line. Therefore, it is unnecessary to invest additionally in the production equipment while the resolution of the LCOS is higher than that of the LCD, which requires glass-manufacturing process.
Liquid crystal on silicon can be categorized into transmissive type and reflective type while major research and development focus on the reflective type. FIG. 1A shows a single pixel on a reflective-type LCOS in a cross-sectional view. The LCOS has a front plate 100 and a rear plate 101. The rear plate 101 includes a silicon substrate 102 on which a thin film transistor 106, an opaque layer 107, and a capacitor 108 are formed. The thin film transistor 106 is used for controlling the operation of the pixel, the opaque layer 107 is used for making the thin film transistor 106 from not being shined so as to avoid misoperation, and the capacitor 108 is used for maintaining the brightness of the pixel. A metal layer 111 is electrically coupled to the thin film transistor 106 and the capacitor 108 while the metal layer 111 is covered with an insulating layer 109. In addition, a pixel electrode 110 is disposed above the insulating layer 109 and is covered with a reflector 112. As to the front plate 100, a glass plate 120 is included and a transparent electrode (indium-tin-oxide electrode) 118 is formed on the glass plate 120. The front plate 100 and the rear plate 101 are assembled in parallel and the space between them is filled with liquid crystal molecules 115 so as to form a liquid crystal layer 114. Further, alignment films 113 and 116 for molecular alignment are formed on the reflector 112 and the transparent electrode 118.
By the above structure, a light signal having brightness corresponding to a voltage applied to the pixel electrode 110 is obtained. When an incident ray (denoted by I, as shown in FIG. 1A) that is normal to the liquid crystal layer 114 strikes the glass plate 120, a reflected ray (denoted by O) is reflected by the reflector 112. The polarization of the light passing through the liquid crystal layer 114 is modulated by changing the alignment of the liquid crystal molecules 115 that is varying with a voltage applied to the pixel electrode 110. After that, the reflected ray is processed by the polarizing film (not shown in FIG. 1A) formed on the glass plate 120. In this way, the polarized reflected ray has the brightness corresponding to the voltage applied to the pixel electrode 110.
FIG. 1B illustrating an LCOS in a top view. As shown, each of the pixel electrodes 110 is isolated with grooves 124, wherein the bottoms of the grooves 124 are covered with the alignment film 113.
To be more specific, when a voltage is applied to the pixel electrodes 110, the arrangement of the liquid crystal molecules is to be varied so that the light transmission changes. Thus, the LCOS can display images with different brightness such as white, black, and intermediate gray scale. In addition, the molecules of the liquid crystal layer of LCOS panels can be categorized into twisted nematic mode (TN) and vertical alignment mode (VA). FIGS. 2A–2B show the operations of liquid crystal molecules in twisted nematic mode when a voltage is not applied or applied to the liquid crystal molecules, respectively. When an electric field is not applied across the alignment films 202 and 204, the liquid crystal molecules 200 gradually twist layer by layer until the uppermost layer is at a 90° angle to the bottom layer, as shown in FIG. 2A. When a sufficient electric field is applied, the liquid crystal molecules 200 are to be aligned and parallel to the direction of the electric field, as shown in FIG. 2B. FIGS. 3A–3B show the operations of liquid crystal molecules in vertical alignment mode when a voltage is not applied or is applied to the liquid crystal molecules, respectively. When a voltage is not applied across the alignment films 302 and 304, the liquid crystal molecules 300 are aligned and perpendicular to the alignment films 302 and 304, as shown in FIG. 3A. When a voltage is applied, the liquid crystal molecules 300, as shown in FIG. 3B, are to be twisted by an angle of 90° to the direction of the liquid crystal molecules 300 when the voltage is not applied, while they are parallel to the alignment films 302 and 304.
As compared with LCOS panels with liquid crystal molecules in twisted nematic mode, LCOS panels with liquid crystal molecules in vertical alignment mode have higher contrast ratios. A twisted nematic LCOS panel can provide a contrast ratio of about 100:1 to 150:1, but a vertical-alignment LCOS panel can provide a contrast ratio of about 400:1 or above. Therefore, the development of LCOS panels with liquid crystal molecules in vertical alignment mode is interested.
Moreover, the liquid crystal layer 114 can be damaged if a voltage in the same polarity is continuously applied to the pixel electrode 110. This problem can be avoided by using polarity inversion because the gray levels produced by the LCOS panel is related to the difference between voltages across the liquid crystal layer 114 but not related to the polarities of the voltages. Polarity inversion is a driving method that a voltage of alternate positive and negative is applied to the pixel electrode 110. With respect to polarity inversion, liquid crystal display driving methods can be categorized into frame inversion, column inversion, and dot inversion. The following is to describe the three driving methods briefly.
FIG. 4A illustrates the conventional frame inversion driving method for a liquid crystal display (LCD) panel 400 having a number of pixels 401. The positive sign “+” and negative sign “−”, hereinafter, are indicative of polarities of the voltages applied to the associated pixels. In frame inversion, if positive voltages are applied to all pixels at one time, then negative voltages are applied to them in the next time instant. In this way, voltages in positive and negative polarities are alternately applied to them.
FIG. 4B illustrates the conventional column inversion driving method for an LCD panel 402 having a number of pixels 403. In column inversion, polarity inversion occurs on pixels of columns. If positive voltages are applied to a column of pixels, negative voltages are applied to the adjacent column of pixels. In the next time instant, the polarities of voltages applied to the above pixels are inverted. That is, negative voltages are applied to the column of pixels that the positive voltages have been applied to, while positive voltages are applied to the adjacent column of pixels that the negative voltages have been applied to. In this way, the application of voltages in positive and negative polarities to the other columns of pixels changes. In this example, the unit that polarity inversion occurs on is one column of pixels. Naturally, this unit can be extended. For instance, two columns of pixels is as a unit for polarity inversion, as shown in an LCD panel 404 of FIG. 4C, and the corresponding driving method is referred to as two-column inversion.
FIG. 4D illustrates the dot inversion driving method for an LCD panel 406 having a number of pixels, viewed as a number of dots. In dot inversion, the polarity of voltage applied to one pixel is the inverse of that applied to the pixels that surround the one pixel. That is, for one pixel that a negative voltage is applied to, voltages in positive polarity are to be supplied to the pixels adjacent to the one on all sides (left, right, top, and bottom sides). In the next time instant, the polarities for every dot are changed.
For an LCD panel with a large size, such as panels used in notebook personal computers, a wide visual angle is achieved by forming multi-domains in every single pixel of the panel. FIGS. 5A and 5B illustrate the arrangement of multi-domain liquid crystal molecules in vertical alignment mode of an LCD panel when a voltage is applied or not applied, respectively. For the sake of brevity, the arrangement of the molecules in a single pixel is described. As shown in FIG. 5A, when no voltage is applied, most of the liquid crystal molecules 500 are aligned vertically to a pixel electrode 502. The pixel electrode 502 has a protrusion 504. The liquid crystal molecules adjacent to the protrusion 504 are arranged substantially vertical to the protrusion 504, and have an inclination to the pixel electrode 502. In addition, the molecules on both sides of the protrusion 504 incline to the both sides. When a voltage is applied, as shown in FIG. 5B, two different domain are formed on the single pixel because of the different inclinations of the molecules on the left and right sides of the protrusion 504. To be more specific, the molecules adjacent to the left side of the protrusion 504 affect the left portion of the liquid crystal molecules 500 of the pixel, so that the left portion of molecules incline to the left side. Likewise, the molecules adjacent to the right side of the protrusion 504 affect the right portion of the liquid crystal molecules 500 of the pixel, resulting in the inclination of this portion of molecules to the right side. FIGS. 5A and 5B show the example with only two domains in one single pixel. However, multiple domains can be similarly implemented by changing the shape of the protrusion 504, leading to a wide visual angle.
As an example, in 1997, Fujitsu limited company produces a multi-domain, vertical alignment mode, thin film transistor (TFT) LCD panel having a visual angle of up to 160°. Since the liquid crystal molecules are in vertical alignment mode, the panel has a contrast ratio of up to 300:1. However, the application of protrusions on the plate of the multi-domain panel results in a reduction in its light efficiency.
Unlike LCD technology for use in large-sized panels, LCOS technology is applied to small-sized panels, e.g., the liquid crystal panels for use in projectors or projection televisions. Besides, their LCOS panels are not required to provide wide visual angles. Instead of relying on the panel to provide wide visual angles, a projector with LCOS can employ an enhanced screen as a scattering surface to achieve wide visual angles. Accordingly, the consideration of wide visual angles in LCOS becomes unnecessary and the LCOS is only required to be capable of being struck by incident rays normal to the LCOS and of reflecting normal reflected rays normal to the LCOS. Thus, the formation of one single-domain in each pixel is sufficient. That is, in the present of an electric field for the LCOS, the liquid crystal molecules in a pixel are inclined to one direction other than multi-directions as illustrated in FIG. 5B.
In brief, for meeting the requirements for the incident light and reflection of the reflected light normal to the LCOS panel, the liquid crystal molecules in one single pixel are desired to be of single-domain and no protrusion, for the pixel, is formed on the plate of the LCOS. In addition, liquid crystal molecules in VA mode are chosen in order to provide high contrast ratios. In the following, under different driving methods, the formation of the single-domain vertical alignment in a LCOS panel and its reflection ratios corresponding to the twisted liquid crystal molecules are illustrated, wherein four pixels are involved. Besides, the alignment films employ the rubbing process to cause the liquid crystal molecules to be arranged on the aligning files in particular directions when no voltage is applied. When a sufficient voltage is applied, the liquid crystal molecules incline to fixed directions so as to form a single-domain in each pixel.
FIG. 6A illustrates a conventional VA mode LCOS when no voltage is applied. Pixel electrodes 602, 604, 606, and 608 are formed on an insulating layer 600, and grooves 610, 612, and 614 are formed among the pixel electrodes. FIG. 6B is a diagram of reflection ratio versus location on the liquid crystal layer corresponding to the four pixels shown in FIG. 6A. Since no voltage is applied, most of the liquid crystal molecules 616 are aligned and vertical to the pixel electrodes 602 to 608, resulting in the LCOS having a reflection ratio of 0%.
FIG. 7A illustrates the LCOS in FIG. 6A driven by the frame inversion driving method. Referring to FIG. 7B also, a diagram of reflection ratio versus location on the liquid crystal layer in FIG. 7A is shown. In order to display images with different brightness, LCD panels are commonly required to produce different gray levels. Thus, voltages of different levels would be applied to adjacent pixels of the LCOS. For instance, the pixel electrodes 602 and 606 are supplied with a voltage of +3.6 V, while the pixel electrodes 604 and 608 with a voltage of +1.5 V. Due to the voltages supplied, the liquid crystal molecules would twist. As examined from FIG. 7A, the higher the voltages applied to the pixels, the larger the inclination angles of the liquid crystal molecules 616. In addition, the dashed lines in FIG. 7A are indicative of equipotential lines yielded after the voltages are applied to the pixel electrodes 602, 604, 606, and 608. By the pattern of the equipotential lines, the distribution of the electric field in the liquid crystal molecules can be determined. Since fringe field effect occurs on the electric field near the edges of the pixel electrodes 602, 604, 606, and 608, the potential lines near the edges of the pixel electrodes 602, 604, 606, and 608 are distributed irregularly. Besides, because of a potential difference of 3.6−1.5 V=2.1 V between adjacent pixel electrodes, the distribution of the electric field near the edges of the pixel electrodes becomes irregular, resulting in the liquid crystal molecules 606 twisting in irregular directions. Thus, reflection ratios of two points a and b located near the edges of the pixel electrodes 602, 604, 606, and 608 reduce to 0%. In other words, while the liquid crystal molecules, under the application of a voltage of +3.6 V, are expected to provide a reflection ratio of about 40 to 50%, an undesired small gray area occurs on the right side of each of the pixel electrodes 602 and 606, thus degrading the display quality of the pixels. Hence, for LCOS panel, the fringe field effect reduces the brightness of the pixels and even produces black points.
FIG. 8A illustrates the LCOS in FIG. 6A driven by the two-column inversion driving method. Referring to FIG. 8B also, a diagram of reflection ratio versus location on the liquid crystal layer in FIG. 8A is shown. When the pixel electrodes 602 and 604 are supplied with a voltage of +3.6 V while the pixel electrodes 606 and 608 are supplied with a voltage of −3.6 V, the liquid crystal molecules 616, originally normal to the pixel electrodes 602 to 608, are to be twisted. However, since the voltages applied to the adjacent pixel electrodes 604 and 606 are in inverse polarities, an electric field in transverse direction occurs between the two adjacent pixel electrode. That is, in the proximity of the groove 612, an electric field occurs in the direction from the pixel electrodes 604 to 606, in parallel to the X-axis. Besides, the fringe field effect of the electric field between the pixel electrodes 604 and 606 on the electric field in the proximity of the groove 612 causes the distribution of electric field to be irregular, resulting in the irregular inclinations of the liquid crystal molecules near the groove 612. Therefore, the reflection ratios, between the pixel electrodes 604 and 608, that are expected to be 40% are now reduced to 0%, such as that indicated by the point c.
FIG. 9A illustrates the LCOS in FIG. 6A driven by the dot inversion driving method. Referring to FIG. 9B also, a diagram of reflection ratio versus location on the liquid crystal layer in FIG. 9A is shown. The pixel electrodes 604 and 608 are supplied with a voltage of +3.6 V while the pixel electrodes 602 and 606 are supplied with a voltage of −3.6 V. Since the adjacent pixel electrodes 602 and 604 (604 and 606; 606 and 608) are supplied with the voltages in inverse polarities, a transverse electric field in the direction of X-axis occurs between the two adjacent pixel electrodes. Besides, the fringe field effect of the electric fields between the pixel electrodes 602, 604, 606, and 608 causes the distribution of the electric fields in the proximity of the grooves 610, 612, and 614, respectively, to be irregular. This results in the regular inclinations of the liquid crystal molecules 616 near the edges of the pixel electrodes 602 to 608. Hence, the reflection ratios of the pixel electrodes 602, 604, 606, and 608 are reduced, and in the worst case, reflection ratios of zero occur on several points, such as points e, f, g, and h, as shown in FIG. 9B.
As can be examined from the performance of the above three driving methods (illustrated by FIGS. 6A–9B) for the conventional LCOS, irregular inclinations occur in the liquid crystal molecules near the pixel electrodes of the conventional LCOS because of the fringe field effect of electric field produced by the pixel electrodes and the effect of transverse electric fields produced by adjacent pixel electrodes supplied with voltages in inverse polarities, e.g., in dot inversion. Thus, the reflection ratios in the regions of the pixels are reduced. In the worst case, black stripes would even occur in the pixel regions, degrading the display quality of the LCOS. Therefore, the loss of light transmission because of irregular molecule arrangement by the transverse electric field and fringe field effect is a critical problem in the development of the single-domain vertical alignment mode LCOS desired to be resolved.